Atomic layer deposition (ALD) processes are useful for the creation of thin films, as described by M. Ritala and M. Leskela in “Atomic Layer Deposition” in Handbook of Thin Film Materials, H. S. Nalwa, Editor, Academic Press, San Diego, 2001, Volume 1, Chapter 2. Such films, especially metal and metal oxide films, are critical components in the manufacture of electronic circuits and devices.
In an ALD process for depositing copper films, a copper precursor and a reducing agent are alternatively introduced into a reaction chamber. After the copper precursor is introduced into the reaction chamber and allowed to adsorb onto a substrate, the excess (unadsorbed) precursor vapor is pumped or purged from the chamber. This process is followed by introduction of a reducing agent that reacts with the copper precursor on the substrate surface to form copper metal and a free form of the ligand. This cycle can be repeated if needed to achieve the desired film thickness.
The ligands coordinated to the metal complex used in the ALD processes must be stable with respect to decomposition and be able to desorb from the complex in a metal-free form. Following reduction of the copper, the ligand is liberated and must be removed from the surface to prevent its incorporation into the metal layer being formed.
In the deposition of metal films by the typical, two-step ALD process, reactivity must be designed into the precursor to enable self-limiting adsorption to the substrate surface. Because a metal surface has no functional groups, this reactivity must be built into the precursor. The initial deposition requires a transformation, such as loss of a ligand, that is induced thermally. The requirement is the source of the conflicting requirements for a metal precursor. The compound must be stable enough to be transferred into the deposition chamber, but reactive enough to undergo a transformation at the substrate surface to form a monolayer of a nonvolatile complex on the substrate ready for the transformation into a pure, conformal metal film.